One type of drug delivery device known in the art is an injection apparatus which contains a medical, therapeutic, diagnostic, pharmaceutical or cosmetic compound (drug) before it is administered, and which is used to administer the compound through the skin of the patient via a hollow needle.
Injection apparatus of this type include pre-filled syringes and autoinjectors. These may contain a drug within a glass or plastic housing that is sealed at one end by a moveable rubber plunger, the internal surfaces of the housing and the plunger having been lubricated prior to being filled with drug by the application of silicone. The rubber plunger performs the dual functions of sealing an opening in the container prior to administration of the drug to the patient, and also of applying a force required to expel the drug from the container during administration of the drug to the patient.
Rubber plungers of this type typically incorporate multiple continuous sealing contact rings spaced apart from each other so that movement of the plunger within the container due to changes in atmospheric pressure during storage does not allow any surface of the container to be in contact at different times with both the drug and the unsterile environment outside the container. The multiple rings also increase the effective thickness of the rubber seal between the drug and the atmosphere outside the housing in order to provide a substantial barrier to gasses and moisture. A disadvantage of these multiple rings is that they increase the force required to move the plunger within the housing.
Other factors which affect the forces required to move the rubber plunger during administration of the drug to a patient can include the distribution and quantity of any silicone or other lubricant present on the surface of the housing or the plunger. In practice these forces can vary greatly due to variation in consistency of lubricant application during manufacture and to displacement of the lubricant and consequent bonding of the plunger to the housing over time. This has a corresponding impact on the design requirements and reliability of the drug delivery device. In addition any lubricants, such as silicone, in contact with the drug can have a detrimental effect on the drug, which can shorten the time for which the drug remains in a usable condition within the device.
Some prefilled syringes and autoinjectors incorporate a second opening within the housing through which the drug is expelled via a hollow hypodermic needle which is fixed to the housing. During storage of the filled device prior to administration of the drug, the needle is blocked by a rubber cap in order to seal the contents of the housing. This design has the disadvantage that it allows the drug to contact the inside of the needle and other materials such as the needle-retaining glue during storage. This design also requires the rubber cap to be removed before the drug is administered, requiring an additional separate action by the device user. This design also typically leaves a relatively large opening in the drug delivery device after the rubber cap has been removed and the drug administered, through which the contaminated needle can be accessed resulting in an increased risk of accidental transmission of blood-borne diseases due to needle stick injury.